Magnetic recording system including magnetic recording medium having three-dimensional random orientation of axis of easy magnetization

ABSTRACT

A magnetic recording system includes a magnetic recording medium including a magnetic layer, the magnetic layer being composed of magnetic grains, and a magnetic head including a recording unit and a reproducing unit. An orientation of an axis of easy magnetization of the magnetic grains is three-dimensionally distributed. An average angle between a direction of an axis of easy magnetization of each of the magnetic grains and a surface of the magnetic recording medium is within a range of 20° to 30°. A squareness ratio Mr/Ms of a remanent magnetization Mr of the magnetic recording medium to a saturation magnetization Ms of the magnetic recording medium is equal to 0.5 or more and is equal to 0.6 or less. A recording density of information which is recorded onto the magnetic recording medium is equal to 360 kfci or more.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 09/435,036filed on Nov. 5, 1999, now U.S. Pat. No. 6,456,448, the contents ofwhich are hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a magnetic recording system suitable for highdensity recording and, more particularly, to a magnetic recording systemhaving a recording density of 10 Gigabits or more per square inch.

2. Description of the Related Art

The realization of a large capacity is required more and more in amagnetic disk apparatus serving as an external magnetic recording systemof a computer. To realize the large capacity, in a magnetic head of themagnetic recording system, a recording unit and a reproducing unit areseparated, an electromagnetic induction type magnetic head is used inthe recording unit, a magneto-resistance effect type head is used in thereproducing unit, and a combination head in which these two heads arecombined is used. According to the magneto-resistance effect type head,since a reproducing sensitivity is higher than that of the conventionalelectromagnetic induction type head, a recording bit becomes fine, andeven if a leakage flux decreases, a sufficiently high reproductionoutput can be obtained. The development of a giant magneto-resistanceeffect type head of a spin valve type having an even higher reproducingsensitivity is also progressing. A magnetic recording medium isconstructed by a Co alloy magnetic layer made of CoCrTa, CoCrPt, or thelike, and a Cr underlayer to control crystal orientation of the magneticlayer. The Co alloy magnetic layer has a hexagonal close packed (hcp)lattice structure in which a c axis is used as an axis of easymagnetization, and it is considered to be desirable that the directionof the axis of easy magnetization is isotropically oriented in the planeof the magnetic recording medium as for an in-plane magnetic recordingmedium, and methods of improving the orientation of the direction of theeasy axis of magnetization in the plane of the magnetic recording medium(JP-A-62-257618, JP-A-63-197018) have been proposed. In the case ofusing the magneto-resistance effect type head as a reproducing head, thefurther reduction of noises than the conventional ones is required forthe medium in order to reproduce not only a signal of the medium butalso the noises at a high sensitivity. The medium noises are mainlycaused by a disturbance of magnetization in a magnetization transitionregion between the recording bits, and the narrowing of such a regioncontributes to the reduction of the medium noises. For this purpose, itis effective to make magnetic particles of the magnetic film of themedium fine. When the magnetic particles are made fine, however, themagnetization thermally fluctuates, and the recorded magnetization isattenuated with the elapse of time. Generally, it is known that as avalue Ku·V/k·T obtained by dividing the product of a magnetic anisotropyconstant Ku and a volume V of a particle by the product of the Boltzmannconstant k and a temperature T decreases, thermal instability increases(P. Lu et al., “Thermal instability at 10 Gbit/in² magnetic recording”,IEEE Transactions on Magnetics, Vol. 30, No. 6, November 1994, pp.4230-4232). Although it is accordingly desirable to use a materialhaving a large Ku to obtain thermal stability, in the conventionalmedium, the larger the value of Ku is, the more a magnetic anisotropymagnetic field Hk increases, so that a coercive force Hc of the mediumalso increases. Generally, however, it is known that, as a magneticfield of the head upon recording, a magnetic field which is 1.5 to 2times as high as Hc is needed at the center of the film thickness of themedium. According to the ability of the current magnetic head, as thecoercive force Hc of the medium increases, recording becomes impossible.It is therefore necessary to realize a medium such that Ku is large sothat thermal stability is obtained, Hc according to the ability of therecording head is obtained, and a high output signal to noise ratio (S/Nratio) is obtained even in high density recording.

SUMMARY OF THE INVENTION

To realize a magnetic recording system suitable for high densitymagnetic recording, the realization of fine magnetic particles of themagnetic layer of the medium is necessary to reduce noises. An influenceby a thermal fluctuation in association with it, however, causes aproblem that magnetization is attenuated with the elapse of time. It issufficient to select a medium material of a large anisotropy constant Kuas one of means for suppressing the influence by the thermalfluctuation. In the conventional technique, the material having large Kusimultaneously shows a high coercive force Hc. However, although anenough large head magnetic field is necessary to record to the medium ofhigh Ku (high Hc), it is becoming difficult to generate an enoughmagnetic field according to the ability of the current head. Accordingto the invention, even if Ku is increased, Hc can be reduced, so thatthe thermal fluctuation can be suppressed and a further high signaloutput to noise ratio (S/N ratio) can be maintained. Particularly, amagnetic recording system suitable to accomplish the recording densityof 10 Gigabits or more (magnetization transition length is equal to 70nm or less) per square inch can be provided.

According to the invention, there is provided a magnetic recordingsystem comprising a magnetic recording medium having a magnetic layerformed on a substrate through an underlayer; a driving unit for drivingthe medium in the recording direction; a magnetic head constructed by arecording unit and a reproducing unit; means for moving the magnetichead relative to the magnetic recording medium; and recording andreproduction signal processing means for performing a signal input tothe magnetic head and an output signal reproduction from the magnetichead, wherein the reproducing unit of the magnetic head is constructedby a magneto-resistance effect type head and has a recordingmagnetization pattern in which a magnetization transition length of themagnetic recording medium is equal to 70 nm or less, the direction of anaxis of easy magnetization of the magnetic particles in the magneticlayer of the magnetic recording medium is three-dimensionallydistributed (an axis of easy magnetization is inclined in the filmthickness direction relative to the film surface), and a squarenessratio (Mr/Ms) which is a ratio of a remanent magnetization Mr to asaturation magnetization Ms which are measured by applying the maximummagnetic field of the magnetic head in the direction of relativemovement between the magnetic head and the magnetic recording medium isset to a value within a range from 0.5 to 0.6, thereby enabling Hc to bereduced even if Ku is increased, so that a thermal fluctuation issuppressed and a higher recording density can be achieved. The reasonwhy Hc can be reduced even if Ku is increased is as follows. Generally,when an angle which is formed by the direction of the axis of easymagnetization and the head magnetic field direction is equal to 45° orless, as such an angle increases, a magnetization reversal occurs in aweak magnetic field. By three-dimensionally distributing the directionof the axis of easy magnetization, the average angle between thedirection of the head magnetic field and the direction of the axis ofeasy magnetization increases. Therefore, even if Ku is large, theincrease in Hc can be suppressed as compared with a medium in which thedirection of the axis of easy magnetization is two-dimensionallydistributed. A degree of the orientation of the axis of easymagnetization is reflected in Mr/Ms, and when Mr/Ms is equal to 0.5,this corresponds to a state in which the direction of the axis of easymagnetization is perfectly three-dimensionally distributed. In thepresent invention, a range of 0.5 to 0.6 as a value of Mr/Ms correspondsto a region having an orientation that is slightly dominant in thein-plane direction as compared to the perfect three-dimensional randomorientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a result in the case where an Mr/Ms (Mr: remanentmagnetization, Ms: saturation magnetization) dependency of an S/N ratio(S: reproduction output, N: noises) of a magnetic recording system ofthe embodiment 1 and S/N ratios of values of Mr/Ms except that of theembodiment are calculated by a Langevin equation and compared;

FIG. 2A is a schematic plan view of the magnetic recording system of theembodiment of the invention, and FIG. 2B is a schematic cross sectionalview taken along the line IIB-IIB′ in FIG. 2A;

FIG. 3 is a schematic solid view showing a cross sectional structure ofa magnetic head in the magnetic recording system of the invention;

FIG. 4 shows a medium shape in the embodiment 1;

FIG. 5 shows the relation between σ and the average value of angles atwhich the axis of easy magnetization is inclined in the film thicknessdirection of the medium in case of assuming that an orientationdistribution of an axis of easy magnetization is expressed byx²+y²+z²/σ=1 (x: component in the track travelling direction of the axisof easy magnetization, y: component in the track width direction, z:component in the film thickness direction);

FIG. 6 shows the relation between σ and Mr/Ms (Mr: remanentmagnetization, Ms: saturation magnetization) in case of assuming thatthe orientation distribution of the axis of easy magnetization isexpressed by x²+y²+z²/σ=1 (x: component in the track travellingdirection of the axis of easy magnetization, y: component in the trackwidth direction, z: component in the film thickness direction);

FIG. 7 shows a result in the case where an Mr/Ms (Mr: remanentmagnetization, Ms: saturation magnetization) dependency of a changeratio of a reproduction output after the elapse of 100 hours of thereproduction output in the magnetic recording systems of the embodiment1 and except that of the embodiment are calculated by a Monte Carlomethod and compared;

FIG. 8 shows results in the case where time decay of the reproductionoutput in magnetic recording systems in the embodiments 2 and 3 iscompared with those of the conventional magnetic recording system;

FIG. 9 shows results in the case where the recording density dependencyof the S/N ratio in the magnetic recording system in the embodiment 2 iscompared with that of the conventional magnetic recording system; and

FIG. 10 shows results in the case where the recording density dependencyof the resolution in the magnetic recording system in the embodiment 2is compared with that of the conventional magnetic recording system.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1

FIGS. 2A and 2B respectively show a schematic plan view and a schematiccross sectional view of a magnetic recording system of an embodiment 1.The system is a magnetic recording system having a well-known structurecomprising a magnetic recording medium 11; a driving unit 12 forrotating the magnetic recording medium 11; a magnetic head 13; drivingmeans 14 for driving the magnetic head 13; and a recording andreproduction signal processing means 15 for the magnetic head. FIG. 3shows a schematic diagram of a structure of the magnetic head of themagnetic recording system. The magnetic head is a recording/reproductionseparation type head in which an electromagnetic inductive type magnetichead for recording formed on a magnetic head slider substrate 27 and amagneto-resistance effect type head for reproduction are combined. Therecording magnetic head is an inductive type thin film magnetic headcomprising a pair of recording magnetic poles 21 and 22 and coils 23which intersect them. The reproducing magnetic head is amagneto-resistance effect type head comprising a magneto-resistanceeffect sensor 24 and a conductive layer 25 serving as an electrode. Agap layer and a shield layer between the recording magnetic poles areomitted in FIG. 3. A head gap length gl of the electromagnetic inductivetype magnetic head for recording is equal to 0.2 μm and a shieldinterval is equal to 0.2 μm. Under such system conditions, in order toclarify medium magnetic characteristics and a medium structure which cansolve the above problems, various examinations were made by using amagnetic recording simulator. Particularly, the relations among adistribution of an axis of easy magnetization of the medium, recordingand reproducing characteristics, and an aging change of a reproductionoutput were examined, so that it has been found that the problems can besolved by a medium having characteristics which will be explainedhereinbelow. The details are described below. The magnetic recordingsimulator is based on a magnetic recording calculating program using aLangevin equation (W. Brown, Jr., “Thermal Fluctuations of aSingle-Domain Particle”, Physical Review, Vol. 130, No. 5, Jun. 1, 1963,pp. 1677-1686) and a calculating program of an aging change of thereproduction output using a Monte Carlo method (Y. Kanai et al.,“Simulation of Magnetic Aftereffect in Particulate Recording Media”,IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, Pp.4972-4974). A particle shape of the medium has an ultrafine hexagonalprism structure having a diameter of 10 nm and a film thickness (δ) of16 nm as shown in FIG. 4 and the particles are arranged without a gap. Amagnetic anisotropy of the particles is a uniaxial anisotropy and anorientation distribution of the axis of easy magnetization is consideredto be an ellipsoid of x²+y²+z²/σ²=1. (x, y, z) respectively denote acomponent in the track travelling direction (down-track direction) ofthe axis of easy magnetization, a component in the track width direction(cross-track direction), and a component in the film thickness direction(medium thickness direction). σ is an arbitrary constant which controlsthe orientation of the axis of easy magnetization relative to the filmthickness direction. FIG. 5 shows results in which a and the averagevalue of angles at which the axis of easy magnetization is inclined inthe film thickness direction are shown as a graph. The angles weremeasured relative to the film surface. Thus, a medium such that as σ iscloser to 0, the axis of easy magnetization is distributed at randomparallel to the film surface and is hardly oriented in the filmthickness direction is obtained. When σ=1, this means that the axis ofeasy magnetization is three-dimensionally distributed at random(three-dimensional random orientation). If σ is larger than 1, thismeans that the orientation distribution of the axis of easymagnetization is deviated toward the film thickness direction. Thesaturation magnetization Ms is set to be constant (Ms=0.65T). Themagnetic anisotropy constant Ku is set to a large value within a rangeof 1.0 to 2.2=10⁵ J/m³ as the average angle of the orientation of theaxis of easy magnetization is inclined in the film thickness directionso that the coercive force Hc and remanent magnetization Mr of themedium are also set to be constant (Hc=250 kA/m, Mr=0.35T, Mr·=5.6 Tnm).The magnitude Hc=250 kA/m of the coercive force is a value obtained byassuming that as a result that the head structure is drawn by aschematic diagram and a head magnetic field is measured by athree-dimensional magnetostatic field analysis simulator, since themaximum magnetic field intensity is equal to 500 kA/m, if the coerciveforce has a value of ½ of it, recording can be sufficiently performed.Under the above conditions, the relation between the parameter a whichcontrols the inclination of the axis of easy magnetization and Mr/Ms isobtained and the result is shown in FIG. 6. It has consequently beendiscovered that as the average angle of the axis of easy magnetizationhas a larger inclination in the film thickness direction, namely, as theaxis of easy magnetization is more three-dimensionally distributed,Mr/Ms decreases. Recording and reproducing characteristics and thermalfluctuating characteristics are calculated by using the medium havingthe above magnetic characteristics and the foregoing head. A spacingbetween the head and the medium at the time of recording/reproduction isset to 45 nm. FIG. 1 shows results of calculations with respect to therelation between the ratio (S/N) of the reproduction output to noisesand Mr/Ms in case of a recording density of 360 kfci (kilo flux changesper inch) (magnetization transition length=70 nm). Thus, when Mr/Ms lieswithin a range from 0.5 to 0.6, the S/N ratio has almost the peak value,and when Mr/Ms is equal to 0.5 or less or is equal to 0.6 or more, theS/N ratio decreases. Further, when the recording density is equal to 360kfci, although an S/N ratio of 20 dB or more is needed, it has beenfound that this condition is satisfied when 0.5≦Mr/Ms≦0.6. subsequently,FIG. 7 shows the reproduction output just after recording in the casewhere recording and reproduction are performed at a recording density of360 kfci (magnetization transition length=70 nm) and a change (%) of thereproduction output after the medium was left at room temperature (300K)for 100 hours (time-decay of the reproduction output). Thus, it has beenfound that when Mr/Ms is equal to 0.6 or less, the reproduction outputhardly changes, and when Mr/Ms is equal to more than 0.6, thereproduction output decreases. This is because it is considered thatsince Ku decreases in association with an increase in Mr/Ms, a thermalstability factor Ku·V/k·T (Ku: magnetic anisotropy constant, V: volumeof a particle, k: the Boltzmann constant, T: temperature) alsodecreases, and the recording magnetization is influenced by the thermalfluctuation and is demagnetized. Consequently, at the recording densityof 360 kfci, by using the medium in which the axis of easy magnetizationof particles is three-dimensionally distributed so as to have an averageangle of inclination of 20° to 30° (Mr/Ms lies within a range Of 0.5 to0.6) in the film thickness direction relative to the film surface, apreferred system which has an excellent thermal stability resistance andan S/N ratio which is equal to 20 dB or more can be obtained. Thisinvention assumes embodiment 1. A point that the calculation resultsalmost match the experimental results is shown in the next embodiments 2and 3.

Embodiment 2

In a magnetic recording system of an embodiment 2, the head having thesame specification as that of the embodiment 1 is used.

The magnetic recording system is formed by using an RF magnetronsputtering method. A glass substrate is used as a substrate, a substratetemperature is set to 280° C., and an underlayer made of SiO₂ or Al₂O₃having a thickness of 0.1 gm is formed on the substrate. Subsequently,an argon gas of 5 mTorr is introduced, an electric power of 0.7 kW/cm²is applied, and a magnetic layer made of CoCrPt having a thickness (δ)of 16 nm is formed. A carbon film having a thickness of 6 nm is furtherformed as a protection layer on the magnetic layer. Magneticcharacteristics of the medium formed in this manner are measured byusing a VSM (vibration sample type magnetometer) by applying a recordingmagnetic field of the magnetic head to the medium in the direction ofrelative motion between the magnetic head and the magnetic recordingmedium. In the case of using SiO₂ as an underlayer, the saturationmagnetization Ms=0.65T, the remanent magnetization Mr=0.35T, and thesquareness ratio Mr/Ms=0.54. Therefore, the product Mr·δ of the filmthickness and the remanent magnetization is equal to Mr·δ=5.6 Tnm. Thecoercive force Hc is equal to 250 kA/m by the measurement using the sameVSM. A magnitude of the magnetic anisotropy constant Ku obtained by themagnetic torque method is equal to 1.8×10⁵ J/m³ and is larger than thatof the conventional medium. The value obtained by dividing the sum ofthe observation particle areas of the medium particles by the number ofobservation particles is set to an average area (=100 nm²) of thecrystal particles and a thermal stability factor Ku·V/k·T is obtained byusing a TEM (Transmission Electron Microscope), so that it is equal to70 at room temperature (T=300K). As a result of examining diffractionpeaks by using a reflection X-ray diffraction method, although peakswere found in both of the [110] orientation and the [001] orientation, aparticularly dominant orientation was not found. Therefore, it has beenconfirmed that the medium in which the direction of the axis of easymagnetization of the magnetic particles of the magnetic layer isthree-dimensionally distributed is obtained. Magnetic characteristics ofthe medium formed by using Al₂O₃ as an underlayer are similarly measuredby the VSM, so that the saturation magnetization Ms=0.66T, the remanentmagnetization Mr=0.35T, and the squareness ratio Mr/Ms=0.53. Therefore,the product Mr·δ of the film thickness and the remanent magnetization isequal to Mr·δ5.6 Tnm. The coercive force Hc is equal to 260 kA/m by themeasurement using the same VSM. A magnitude of the magnetic anisotropyconstant Ku obtained by the magnetic torque method is equal to 1.8×10⁵J/m^(3.) The value obtained by dividing the sum of the observationparticle areas of the medium particles by the number of observationparticles is set to an average area (=100 nm²) of the crystal particlesand a thermal stability factor Ku·V/k·T is obtained by using a TEN(Transmission Electron Microscope), so that it is equal to 70 at roomtemperature (T=300K). As a result of examining diffraction peaks byusing a reflection X-ray diffraction method, although peaks were foundin both of the [110] orientation and the [001] orientation, aparticularly dominant orientation was not found. Therefore, it has beenconfirmed that the medium in which the direction of the axis of easymagnetization of the magnetic particles of the magnetic layer isthree-dimensionally distributed is obtained.

Comparison results obtained by measuring the time-decay of thereproduction output and the recording and reproducing characteristics byusing the magnetic recording system of the invention and theconventional magnetic recording system are shown. The magnetic recordingsystem of the invention is a system comprising the medium using SiO₂ asan underlayer shown above and the recording/reproduction separation typehead which is formed by combining an electromagnetic inductive typemagnetic head for recording having a head gap length gl=0.2 μm and amagneto-resistance effect type head for reproduction having a shieldinterval of 0.2 μm. The spacing between the head and the medium is setto 30 nm. In the conventional magnetic recording system, the same headas that of the invention is used and the conventional medium in whichthe axis of easy magnetization of the particles is distributed in theplane and which is isotropic in the plane is used. As a thickness ofmagnetic layer, a particle diameter, and Mr·δ (=6.0 Tnm) and Hc (=250kA/m) among the magnetic characteristics, almost the same values asthose of the medium used in the magnetic recording system of theinvention are used. However, since Ms=0.5T, the remanent squarenessratio Mr/Ms 0.7, Ku 1.4×10⁵ J/m³, and Ku·V/k·T=54, it is presumed thatthe thermal decay are worse than those of the medium used in theinvention. FIG. 8 shows a time-decay of the reproduction output at alinear recording density of 400 kfci. It has consequently been foundthat the reproduction output deteriorates by 7% in the conventionalmagnetic recording system after 100 hours and that it hardlydeteriorates in the magnetic recording system of the invention. Sincethe time-decay is larger as the magnetic recording density is higher, itis possible to presume that there is hardly a time-decay after 100 hoursat the recording density of 360 kfci lower than that of 400 kfci of theinvention. That is, the calculation results of the embodiment 1preferably coincide with the experiment results. Subsequently, therecording and reproducing characteristics of the magnetic recordingsystem are compared. FIG. 9 shows a dependency of the S/N ratio of thereproduction output to the noises on the recording density. It has,consequently, been known that although the S/N ratio of the magneticrecording system of the invention at the low recording density is lowerthan that of the conventional magnetic recording system, at a recordingdensity of 300 kfci or more, the S/N ratio of the system of theinvention is higher. It has also been found from the diagram that S/N=20dB at the recording density of 360 kfci (magnetization transitionlength=70 nm) and is almost equal to the S/N ratio of the embodiment 1.That is, it has also been found from this result that the calculationresults of the embodiment 1 preferably coincide with the experimentresults. Further, in case of the recording density of 360 kfci, althoughthe S/N=20 dB or more is necessary for the system, in the conventionalsystem, the limit of the recording density is equal to 330 kfci and 10Gigabits per square inch cannot be accomplished. However, in the systemof the embodiment, the recording density is equal to 360 kfci and therecording of 10 Gigabits per square inch can be performed. FIG. 10 showsa dependency of the resolution on the recording density. The resolutiondenotes a value in which an output E_(2f) of the high recording densityis normalized by a reproduction output E_(1f) at the low recordingdensity (50 kfci). Although the reproduction output decreases inassociation with an increase in recording density, in the magneticrecording system of the invention, a decreasing rate is small. When therecording density is equal to 200 kfci or more, it has been found thatthe resolution as a ratio of the output E_(1f) of the low recordingdensity and the output E_(2f) of the high recording density is higherthan that of the conventional system. Particularly, when the recordingdensity is equal to 360 kfci or more, since the resolution of 20% ormore is necessary, the above conditions cannot be satisfied in case ofsystems other than the system according to the invention. It hasconsequently been found that the magnetic recording system of theinvention is thermally stable and suitable for the high density magneticrecording in which the recording density is equal to 360 kfci or more,namely, the magnetization transition length is equal to 70 nm or less.Further, the time-decay and recording and reproducing characteristics ofthe reproduction output of the magnetic recording system using themedium in which the underlayer is made of Al₂O₃ are measured, so thatresults which are almost equivalent to those of the magnetic recordingsystem using the medium in which the underlayer is made of SiO₂ areobtained. It has also been found that the system is thermally stable ascompared with the conventional magnetic recording system and is suitablefor the high density magnetic recording.

Embodiment 3

An embodiment 3 shows a comparison example examined while changing Mr/Msfor the embodiment 2. In a magnetic recording system of the embodiment3, the same head as that of the embodiment 1 is used and as a medium,the same glass substrate, magnetic layer, and protection layer as thosein the embodiment 2 are formed under substantially the same film formingconditions as those of the embodiment 2 except that a Zr or Al filmhaving a thickness of 0.1 μm is provided as an underlayer. According tothe medium formed by using Zr as an underlayer, the saturationmagnetization is equal to Ms=0.60T, the residual magnetization measuredby the VSM is equal to Mr=0.32T, and the remanent squareness is equal toMr/Ms=0.53. According to the measurement by the same VSM, the coerciveforce is equal to 280 kA/m. The magnitude of the magnetic anisotropyconstant Ku obtained by the magnetic torque method is equal to 1.7×10⁵J/m³ and is larger than that of the conventional medium. The magneticcharacteristics of the medium formed by using Al as an underlayer arealmost the same as those of the medium using Zr as an underlayer.Subsequently, the value obtained by dividing the sum of the observationparticle areas of the medium particles by the number of observationparticles is set to the average area (=100 nm²) of the crystal particlesand Ku·V/k·T serving as a thermal stability factor is obtained by usingthe TEM (Transmission Electron Microscopy), so that it is equal to 66 atthe room temperature (T=300K). Even in this magnetic layer, the mediumin which the direction of the axis of easy magnetization of the magneticparticles of the magnetic layer is 3-dimensionally distributed isobtained in a manner similar to the embodiment 2. The time-decay of thereproduction output is examined in a manner similar to the embodiment 2,so that the output after the elapse of 100 hours is reduced by about0.5% as shown in FIG. 8. Further, as a comparison example, thetime-decay and S/N ratio of the reproduction output of the magneticrecording system using the same head as that of the embodiment 2 and themedium having substantially the same layer structure and formed undersubstantially the same film forming conditions as those in theembodiment 2 except that a Ti film having a thickness of 0.05 μm is usedas an underlayer are measured. Although the magnetic characteristics ofthe medium used for comparison exhibit almost the same values, namely,Mr·δ=5.6 Tnm, Hc=240 kA/m, and Ku·V/k·T=75 as those in the embodiment 1by the VSM measurement, Ms=0.87 and the remanent squareness is equal toMr/Ms=0.4, so that the remanent squareness fairly deteriorates. Althoughthe time-decay of the reproduction output is almost the same as that inthe embodiment 2, since the noises increase in association with thedeterioration of the output as shown in FIG. 9, the SIN ratio fairlydeteriorates. It has, therefore, been found that in case of the remanentsquareness is equal to 0.5 or less, it is not suitable for the highdensity magnetic recording.

According to the magnetic recording system using the magnetic recordingmedium in which the direction of the axis of easy magnetization of themagnetic particles in the magnetic layer of the invention isthree-dimensionally distributed as mentioned above, in the case wherethe recording magnetization pattern in which the magnetizationtransition length of the magnetic recording medium is equal to 70 nm orless is recorded, a medium having high Ku and low Hc can be realized.Excellent characteristics for the S/N ratio, resolution, and thermalstability can be obtained.

What is claimed is:
 1. A magnetic recording system comprising: amagnetic recording medium including a substrate, an underlayer formed onthe substrate, and a magnetic layer formed on the underlayer, themagnetic layer being composed of magnetic grains; and a magnetic headincluding a recording unit and a reproducing unit; wherein theunderlayer includes SiO₂ or Al₂O₃; wherein an orientation of an axis ofeasy magnetization of the magnetic grains is three-dimensionallydistributed; and wherein a recording density of information which isrecorded onto the magnetic recording medium is equal to 360 kfci ormore.
 2. A magnetic recording system comprising: a magnetic recordingmedium including a substrate, an underlayer formed on the substrate, anda magnetic layer formed on the underlayer, the magnetic layer beingcomposed of magnetic grains; and a magnetic head including a recordingunit and a reproducing unit; wherein the underlayer includes SiO₂ orAl₂O₃; wherein an orientation of an axis of easy magnetization of themagnetic grains is three-dimensionally distributed; and wherein theorientation of the axis of easy magnetization of the magnetic grains hasa three-dimensional distribution defined by the following equation: x ²+y ² +z ₂/σ²=1 where x is a component of the axis of easy magnetizationin a track travelling direction of the magnetic recording medium, wherey is a component of the axis of easy magnetization in a track widthdirection of the magnetic recording medium, where z is a component ofthe axis of easy magnetization in a medium thickness direction of themagnetic recording medium, and where σ is an arbitrary constant whichcontrols the orientation of the axis of easy magnetization of themagnetic grains relative to the medium thickness direction.
 3. Amagnetic recording system comprising: a magnetic recording mediumincluding a substrate, an underlayer formed on the substrate, and amagnetic layer formed on the underlayer, the magnetic layer beingcomposed of magnetic grains; and a magnetic head including a recordingunit and a reproducing unit; wherein the underlayer includes SiO₂ orAl₂O₃; wherein an average angle between a direction of an axis of easymagnetization of each of the magnetic grains and a surface of themagnetic recording medium is within a range of 20° to 30°; and wherein arecording density of information which is recorded onto the magneticrecording medium is equal to 360 kfci or more.
 4. A magnetic recordingsystem comprising: a magnetic recording medium including a substrate, anunderlayer formed on the substrate, and a magnetic layer formed on theunderlayer, the magnetic layer being composed of magnetic grains; and amagnetic head including a recording unit and a reproducing unit; whereinthe underlayer includes SiO₂ or Al₂O₃; wherein an average angle betweena direction of an axis of easy magnetization of each of the magneticgrains and a surface of the magnetic recording medium is within a rangeof 20° to 30°; wherein respective orientations of respective axes ofeasy magnetization of the magnetic grains have a three-dimensionaldistribution; and wherein the three-dimensional distribution of therespective orientations of the respective axes of easy magnetization ofthe magnetic grains is defined by the following equation: x ² +y ² +z²/σ²=1 where x is a component of the respective axes of easymagnetization of the magnetic grains in a track travelling direction ofthe magnetic recording medium, where y is a component of the axes ofeasy magnetization of the magnetic grains in a track width direction ofthe magnetic recording medium, where z is a component of the axes ofeasy magnetization of the magnetic grains in a medium thicknessdirection of the magnetic recording medium, and where σ is an arbitraryconstant which controls the respective orientations of the axes of easymagnetization of the magnetic grains relative to the medium thicknessdirection.